Smurf
The answer is that any aircraft will fly any length of sector subject to (a) pax demand (they use 747's on Japanese domestic sectors) (a) opeational requirements (BA have use 747's, 777's, Tristars on CDG in the past).
EGLL-EGPF [PH=Posh Houses=EDI, PF=p0xy flats=GLA, PD=pretty dull=ABZ] is about 600km, whereas EGCC=MAN is about 300km.
A 737/320 will burn about 10kg/nm so about 1600kg on a LHR-MAN sector - I would guess a 757 would burn about 14kg/nm, so 40% more at 2200-2300kg.
You do the maths fot the longer sector.
[The 737-400 may burn 10% more than the 320].